The neuropathological hallmarks of Alzheimer's disease (AD) are amyloid plaques and neurofibrillary tangles. Increased phosphorylation of proteins notably tau on serine orthreonine preceding a proline (pSer/Thr-Pro) precedes tangle formation and neurodegeneration. Phosphorylation of APP on the Thr668- Pro motif is shown to regulate Ap secretion in vitro, and to be elevated in AD brains. Recently, we found that pSer/Thr-Pro motifs exist in the two distinct cis and trans conformations, whose conversion is drastically slowed down by phosphorylation, but is catalyzed specifically by the prolyl isomerase Pin1. Pin1 binds to certain proteins and regulates their structure and function by isomerizing specific pSer/Thr-Pro motifs. Importantly, Pin1 is highly expressed in most neurons, but is especially low in vulnerable or degenerated neurons in AD. Pin1 binds to and isomerizes the pThr231-Pro motif in tau, thereby restoring its function and promoting its dephosphorylation. As a result, Pin1 knockout in mice causes age-dependent tauopathy phenotype and neurodegeneration. Moreover, we have now shown that Pin1 has profound effects on APP processing and Ap production. Pin1 binds to the pThr668-Pro motif in APP and greatly accelerates its isomerization, regulating the APP intracellular domain between two distinct conformations as visualized by NMR. Whereas Pin1 overexpression reduces Ap secretion from cell cultures, knockout of Pint increases its secretion. Pin1 knockout alone or in combination with overexpression of mutant APP in mice increases amyloidogenic APP processing and selectively elevates insoluble Ap42 in brains in an age-dependent manner, with Ap42 being prominently localized to multivesicular bodies of neurons, as shown in Alzheimer's disease before plaque pathology. Thus Pin1-catalyzed prolyl isomerization is a novel mechanism to regulate APP processing and Ap production, and its deregulation may link both tangle and plaque pathologies. However, it remains unknown how Pin1 regulates APP processing and Ap production and whether Pin1 affects the plaque pathology. To address these questions, Aim 1 will use molecular and cell biology approaches to determine the mechanisms by which Pin1 regulates APP processing and Ap secretion. Aim 2 will use mouse models to examine the role of Pin1 in the generation of Ap and amyloid pathology. These studies would provide novel insights into the development of AD and related diseases, and may also have novel therapeutic implications. In lay language, we have recently identified a new enzyme important for the development of Alzheimer's disease. In this proposal, we will continue to investigate how this enzyme affects the disease processes and hope to eventually identify new therapeutic targets.